Scandium, in spite of its relative abundance in the earth's crust and attractive properties, is not a commonly used element, partly because there are no convenient (high grade) sources or ores of the metal. Important low grade sources include uranium tailings and the waste sludges of tungsten recovery plants. These sludges consist largely of iron and manganese and other hydrous oxides and contain varying levels of scandium, typically in concentrations of 100-1000 ppm.
Processes which attempt to recovery scandium from such a material face the problem that the waste is extremely complex and heterogeneous chemically. About two dozen other elements are present in greater or lesser amounts. Furthermore, the low level of scandium necessitates the processing of large quantities of material. Consequently, to be economical a recovery scheme must be simple and selective. Ideally, the process should be operable on a continuous as opposed to a batch basis.
Procedures for the separation of scandium from iron and other metals disclosed in issued U.S. patents have disadvantages which make them ill-suited for the large scale production. For instance, U.S. Pat. No. 3,013,859 discloses an extraction based procedure requiring that an aqueous phase be brought to a concentration of 2.5 lbs/gal magnesium nitrate before equilibration with an alkyl phosphate extractant occurs. On a large scale such a procedure would be expensive because of the amount of magnesium nitrate required to process thousands of gallons of solution. A further complication would be the disposal of recovery of this magnesium nitrate.
Another procedure is disclosed in U.S. Pat. No. 2,874,039 which achieves the separation of scandium from iron and other metals by volatilization of the chlorides in a furnace at about 1000.degree. C. Not only would such a procedure be high in energy consumption and low in throughput, it suffers from the further disadvantage that it cannot be operated as a continuous process and it is encironmentally objectionable.
The prevalent approach to the extraction of metal ions from solution by ion exchange resins is the use of strong cation type resins. This technique is useless for the recovery of a small amount of an ion such as scandium in the presence of a large excess of other ions because strong cationic resins adsorb all metallic ions so that the resin becomes quickly saturated and no selectivity is obtained.
In a recent presentation at the 17th Rare Earth Research Conference, McMaster University, Hamilton, Ont., June 9-12, 1986, L. A. Herchenroeder et al., describe the use of a strong cation exchange resin for the purification of scandium oxide. In this application, scandium oxide of 98% purity is dissolved and passed through an ion exchange column packed with a resin of a strong cation type on which all the cations present in solution are adsorbed. The separation of scandium from the other ions is obtained by ion exchange chromatography in which the adsorbed ions are eluted by a reagent which differentiates between different size ions and the eluate is passed repeatedly through twelve ion exchange columns to obtain a scandium band. This takes weeks, if not months, and is done at the impractical high temperature of 96.degree. C. which requires the columns to be placed in a special hot box.
It is, therefore, the purpose of this invention to provide a method for the recovery of scandium from solution using cation exchange resin of the weakly acidic type.
Another object of this invention is to provide a method for the selective adsorption and recovery of trace amounts of scandium in the presence of a large excess of other ions.
Still another object of this invention is to provide a method for the recovery of scandium at room temperature and in a short period of time.
Yet another purpose of this invention is to provide a method for the recovery of scanadium which is cost effective in that it can be made continuous, it is readily adaptable to industrial implementation and is using standard, available reagents.